The primary focus of this paper is to highlight the results of the analytical heat transfer calculations and the computational analysis of the mixing duct of gas generator test rig. The main objective of the gas generator test rig is to simulate the gas-turbine engine exhaust gas flow at the required mass flow rate and temperature conditions. At first, the high temperature gas is generated by burning the Jet-A fuel with air flow in the gas-turbine burner. Then, the high temperature gas from the burner is mixed with cooling air in the mixing duct to get the required mass flow and temperature conditions at the exit of gas generator test rig. Four different conditions of air mass flow distribution between the burner and cooling air flow are considered. Analytical heat transfer calculations are carried out for all the four different configurations by considering all the three modes of heat transfer using the inhouse developed code. The amount of heat loss from mixing duct to the surroundings is calculated for all the four different conditions of mass flow distributions. Adiabatic CFD simulations are carried out to capture the typical flow features of cooling air entry into mixing duct. Penetration and upstream propagation of cooling air entry, temperature distribution of hot and cold air and pressure loss factor are captured from the results of CFD simulations. From the results of analytical heat transfer calculations and computational analysis, optimum condition of mass flow distribution between the burner and cooling air flow is identified. Experimental performance evaluation of gas generator test rig is carried out for the identified condition of mass flow distribution. Marginal deviations are observed between analytical, computational and experimental results of temperature variation along the mixing duct.